The Hydra. 



15 



both "unicellular." The two layers of cells are known 

 respectively as the ectoderm and the endoderm, the names referring 

 to their relative positions. The enteron, or central cavity, is also 



Fig. 6.— Longitudinal section through Hydra. (After Marshall and Howes.) 

 The asterisks (^) are placed in enteron. 



continued into the tentacles. The ectoderm is made up of two 

 kinds of cells ; there are, in the first place, large cells somewhat 



